In general, a semiconductor laser is a diode device in which a forward bias voltage is applied across an active layer, which is surrounded by one or more waveguide layers, which in turn are surrounded by a pair of cladding layers. One cladding layer is an n-doped layer and the other is a p-doped layer so that excess electrons from the n-doped layer and excess holes from the p-doped layer are injected through the wavelength layers and into the active layer by the bias voltage, where they recombine. At current levels above a threshold value, stimulated emission occurs and a monochromatic, highly-directional beam of light is emitted from the active layer. A resonant cavity is formed in the waveguide layers at one end of the device by a highly-reflective surface and at the other end of the device by a partially-reflective surface through which the beam emerges. The cladding layers usually have a lower index of refraction than the active and waveguide layers which transversely confines the laser light to the active and waveguide layers.
One technique to laterally confine the optical output of a semiconductor laser diode for operation at the fundamental lateral mode is gain guiding. Gain guiding utilizes an electrical contact to supply current to the device. This contact defines the lasing region of the device. However, at high power levels, gain-guided laser diodes have strong instabilities and generate broad, highly-astigmatic multi-peaked beams. Further, gain guiding is generally only operable for lateral waveguides having widths in the 100's of microns range.
Another technique for laterally confining the optical output of a semiconductor laser diode is index guiding. Index guiding employs various dielectric waveguide structures to laterally confine the laser light. These waveguide structures are either positive-index guides, in which the index of refraction is higher in the region aligned with the laser element and lower in the regions surrounding the laser element, or negative-index guides, in which the index of refraction is lower in the region aligned with the laser element and higher in the regions surrounding the laser element. Positive-index guiding effectively traps light in the laser element, while negative-index guiding, or antiguiding, allows light to leak out of the lasing element.
Conventional positive-index guided laser diodes have aperture spot sizes on the order of 2 to 3 microns and operate reliably at single-mode power levels of less than 200 mW. Such sizes and power levels can be inadequate for some applications, such as pumping optical amplifiers for long range transmission.
A high optical power density at the output facet of a semiconductor laser negatively affects both the maximum attainable optical power from the laser before the onset of catastrophic optical mirror damage (COMD) and the long-term reliability of the device. A device in which the optical mode is confined to a narrow region will exceed the maximum power density of the output facet at a lower total output power than a device in which the optical mode is distributed over a relatively large area. When devices of both types are operated at the same total output power the higher power density in the device with the narrow optical mode will contribute to greater local facet heating and therefore accelerated facet degradation relative to the device with the wider optical mode, leading to reduced device lifetimes and reliability. Although these considerations suggest that such devices should have the widest possible optical mode, care must also be exercised to ensure that single mode operation is maintained if the device is to be coupled to a single mode fiber. If a laser can support undesirable modes above threshold in addition to the desired mode, coupling inefficiencies and kinked ex-fiber power-current relationships can occur. For such applications, it would be highly desirable to have a laser design that would exhibit properties such as low optical power density and single mode operation in addition to being manufacturable from both a materials growth and processing tolerance perspective.
Several attempts have been made to increase the size of the lateral optical mode (lateral spot size) to lower the optical power density at the output facet. Tapered waveguides attempt to expand the beam at the output facet of the laser thus producing a larger optical mode. See T. L. Koch, U. Koren, G. Eisenstein, M. G. Young, M. Oron, C. R. Giles, B. I. Miller, “Tapered Waveguide InGaAs/InGaAsP Multiple Quantum Well Lasers,” IEEE Photonics Technology Letters, 1990, pp. 88-90. However, the techniques required to construct a tapered structure are not easily incorporated into a standard MOCVD regrowth manufacturing process. Broad lateral waveguides may offer potentially low power density modes but may also support higher-order modes at high drive current. ARROW structures can support large lateral single mode. See D. Botez, Iulian Basarab Petrescu-Prahova, Luke J. Mawst, “High Power Laterally Antiguided Semiconductor Light Source with Reduced Transverse Optical Confinement,” U.S. Pat. No. 6,167,073, Dec. 26, 2000.